Raft Mat Foundations: Design, Applications, and Requirements

Content

• Combined Footing Design
• Purchase the our Soil Bearing Capacity and Settlement Calculation Bundle for only £25.
• Sustainable Construction Methods and Their Benefits
• 1.1 Cover to Reinforcement
• First Carbon Neutral Spa Retailer in the world!
• Spa Pools
• Spa pool regulations in Australia 2025 (Rules, requirements and more)
• What Are Monolithic Slabs?

The design must consider naturally occurring groundwater relative to the water table in the area, and also the possibility of surface water infiltration on the site. If groundwater is present at excavation depth, dewatering methods will be necessary to facilitate construction. Once the pool is in place, groundwater levels that are above the lowest elevation of an in-ground pool structure will exhibit buoyant forces on the pool shell. If groundwater levels are naturally high on the site relative to the foundation elevation, a subsurface drainage system to remove groundwater from the soil surrounding the pool shell may be necessary.

Prolonged exposure to freezing temperatures can literally freeze the water in the soil’s voids, leading to frost heave. This condition can lead to significant soil expansion that can severely damage foundations that weren’t designed correctly. Once again, we’ll share more about how to deal with this in a later section of this article. Whether you’re building a container pool, a container house, or just placing an empty container on your property for storage, you need to safely and securely support it.

Purchase the our Soil Bearing Capacity and Settlement Calculation Bundle for only £25.

Even a poor soil—silt or soft clay—might have an allowable soil pressure of 400 pounds per square foot. Another perhaps slightly less common approach to dealing with swell potential of expansive soils is to moisture condition the soils in order to minimize the possibility of volume change occurring during or following construction. The goal with this method is to saturate the expansive soils prior to construction and maintain the moisture content throughout not only the project, but theoretically the life of the facility as well. This is only appropriate for certain soil types and generally only those that do not exhibit extreme shrink/swell potential.

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• Sure, it has to be able to support the slab, and on most ground that's not a big problem, at least across the middle of the slab, since the load is spread across so much area.
• The compression of soil beneath the footing occurs suddenly with load increase and the failure surface does not extend above the ground surface.
• This plan should include the pool's location, placement of features like steps, benches, and lights, as well as the pool deck and equipment layout.
• By assigning a resilience modulus (k) to the soil, you can figure out the necessary slab thickness.
• You’ll need to do a lot of digging to get the perimeter of the slab below frost depth, and might be better off building a concrete basement instead.

1.1 Cover to Reinforcement

The construction process for monolithic slabs involving a concrete slab foundation is both straightforward and precise. First, the building site is adequately prepared by clearing the area of vegetation, rocks, and any debris that could interfere with a uniform pour. Unlike traditional foundations, which are constructed in multiple stages, monolithic slabs are poured all at once. This streamlined process explains why many builders favor them for efficiency and cost savings, particularly in stable soil conditions. The thing to remember here is that a concrete slab is intended to be rigid, but we don't expect the base to be infinitely stiff. A slab will settle a little and that's OK from a design standpoint—again, as long as the settlement is uniform.

Tolerances on the base course are +0 inches and minus 1 inch for Class 1 through 3 floors (typical low tolerance floors) or +0 inches and minus ¾ inches for higher tolerance floors. The thicker the subbase, the more load the slab can support, so if there are going to be heavy loads on the slab—like trucks or fork lifts—the designer will probably specify a thick subbase. A subbase can also act as a capillary break, preventing water from wicking up from the groundwater table and into the slab. The subbase material is usually a reasonably low cost gravel without a lot of fines. To calculate the allowable bearing capacity we simply reduce the ultimate bearing capacity by our factor of safety. This factor of safety is variable in different standards and guidelines but generally ranges from a value of 2 to 3.

Spa Pools

But both of those downsides can actually be addressed as this next foundation type shows. Screw piles also provide a huge amount of uplift resistance compared to many other foundation types, which is helpful for securing a container during a storm. And when it’s called an ‘anchor’ versus a screw, pile, or pier, it typically means the intended use is for pull-out resistance. In response, you need to be mindful of soil compaction (discussed later) as well as the presence of significant amounts of organic matter or clay. Either way, this is an area where we strongly recommend getting professional help. Let’s start with organic matter, which typically means things like decaying leaves, animal waste, etc.

• Vibratory force uses some sort of mechanism to vibrate the soil, which reduces the friction between the soil particles, allowing them to squeeze together more easily.
• Still, these slabs might not suit every site, especially those with steep slopes or highly variable soil conditions where deeper footings or separate walls are the norm.
• Typical soil bearing capacity values are useful for preliminary and feasibility design.
• The integral action of the raft—its ability to act as a single solid entity—improves the performance of the foundation under varying loads.
• Attaching a container to a foundation will ensure the container can’t slide laterally or be lifted off the foundation.

In most cases, you'll want your patio to be 4 inches thick, although you should add reinforcement and increase the thickness up to around 8 inches if you want it to support heavier loads and resist cracking more readily. Although thicker slabs will cost more, you'll start seeing diminishing returns at around 6 inches. One of the most commonly used method is plate load test, in which a bearing plate is kept on the soil and load is put on it to calculate the settlement of the plate. A safe settlement is supposed, for example 5 mm, and the load which makes the soil settle 5 mm is calibrated as the bearing capacity of that soil. Concrete footings and slabs on grade typically require a concrete of 3,500 to 4,000 psi.

For anything more than about a cubic yard or two, consider having the concrete delivered directly to your site, ready to use. First is that ProGorki have to be exact, otherwise the container won’t actually fit onto all four twist locks! Second is that twist locks only mate with the container corner fittings, so if you want additional points of contact between the container and foundation, a twist lock can’t be used.